1. Field of the Invention
The present invention relates to a high-side switch driver, and more particularly relates to a driver circuit for driving the high-side switch of a power converter.
2. Description of the Prior Art
Many commonly used power converters utilize bridge circuits to regulate a voltage source in response to a load. Some of these power converters include power supplies and motor drivers.
The bridge circuits used by these power converters are normally built from a pair of switching devices connected in series across the voltage source. The switching devices include a high-side switch and a low-side switch. The high-side switch is connected to the voltage source while the low-side switch is connected to the ground reference. A common node between the high-side switch and low-side switch is coupled to the load. The switches are generally transistor devices (MOSFET, IGBT, etc). The switches are controlled to alternately conduct, so that the common node will periodically swing in between the voltage source and the ground reference. When the high-side transistor is turned on, the voltage of the common node will rapidly shift to the voltage level of the voltage source. When the high-side transistor is fully turned on, the bridge circuit will operate in low-impedance mode. To turn on the high-side transistor, the gate drive voltage must exceed the voltage of the voltage source. Thus, the gate-to-source voltage of the high-side transistor will be floated with respect to the ground reference.
FIG. 1 demonstrates a technique that employs a pulse transformer 5 to create a floating voltage for driving a high-side transistor 10. However, the design of the pulse transformer 5 suffers from two disadvantages. First of all, its size is relatively large. Second, the pulse transformer 5 will require a substantially higher driving current due to magnetizing-current consumption.
FIG. 2 shows another prior-art bridge circuit using a bootstrap capacitor 30 and a charge-pump diode 40 to create a floating voltage for driving a gate of a high-side transistor 10. Switching on a transistor 45 will pull down the voltage at the gate of the high-side transistor 10 to the ground reference. The gate voltage of the high-side transistor 10 will be pulled down via a diode 42. This will turn off the high-side transistor 10. Once the high-side transistor 10 is turned off and a low-side transistor 20 is turned on, the floating voltage of the bootstrap capacitor 30 is charged up to a bias voltage VB via the charge-pump diode 40. Switching off the transistor 45 will turn on the high-side transistor 10 by conducting the floating voltage to the gate of the high-side transistor 10 via a transistor 41. The drawback of this circuit is that the gate voltage level might not be sufficient to ensure proper operation. Since the bootstrap capacitor 30 is charged via the low-side transistor 20, its charge time and the bootstrap voltage may decrease to an unacceptable level due to the reduced duty cycle of the low side transistor 20. Furthermore, the voltage drops across the diode 40 and the transistor 41 will cause the gate voltage level of the high-side transistor 10 to fall.
Recently, various techniques have been proposed for generating the required gate voltage for the high-side transistor in a more reliable manner. One such technique appears in U.S. Pat. No. 5,381,044 (Zisa, Belluso, Paparo), U.S. Pat. No. 5,638,025 (Johnson), and U.S. Pat. No. 5,672,992 (Nadd).
The drawback of these prior-arts is the need for a switch-off transistor such as the transistor 45. The prior-art bridge circuits listed above use something like the transistor 45 of FIG. 2 to turn off the high-side transistor. This switch-off transistor must be manufactured according to a high-voltage process, in order to be safely used in high-voltage applications (200 volts or more). To be integrated into a silicon chip, this high-voltage transistor requires a relatively thick coat of oxide and silicon. Furthermore, the parasitic capacitance of this high-voltage transistor will slow down the slew rate of the switching signal, thus resulting in significant high-side transistor switching losses. Therefore, these prior-arts are not suitable for high-voltage applications, or for high-speed applications.
To remedy this shortcoming, a technique using a boost converter is proposed in U.S. Pat. No. 6,344,959 (Milazzo). The boost converter is essentially a voltage doubling circuit. While this technique generates a more reliable gate voltage to drive the high-side transistor, it requires an additional switching element and other circuitry. This increases the cost and complexity of the driving circuit. Moreover, severe noise will be generated by the voltage source and the ground reference, due to high frequency charging and discharging of the voltage doubling capacitor in the charge pump.
The objective of the present invention is to provide a high-side switch driver for high-voltage and high-speed applications that overcome the drawbacks of prior art high-side switch drivers.
The capacitive high-side switch driver according to the present invention includes an inverter and two totem-pole buffers. The switch driver controls the totem-pole buffers in response to an input signal, in such a manner that they alternately conduct with complementary duty cycles. The outputs of the two totem-pole buffers drive two high-voltage capacitors. These high-voltage capacitors are further coupled to the input of a high-side circuit. The high-side circuit comprises a comparator, a programmable load, an under-voltage protector, and a drive-buffer for driving a high-side transistor. The high-side circuit further consists of a charge-pump diode and a bootstrap capacitor.
When the low-side transistor is turned on, the bootstrap capacitor is charged to drive the high-side transistor. The two totem-pole buffers and the two high-voltage capacitors generate differential signals to drive the comparator, and further charge the bootstrap capacitor via a bridge-rectifier. The bootstrap capacitor is used to supply power for the high-side circuit.
One objective of the present invention is to provide protection against low gate-voltage levels. The under-voltage protector enables the drive-buffer whenever the floating voltage exceeds the start-threshold voltage, and disables the drive-buffer whenever the floating voltage drops below the stop-threshold voltage. The under-voltage protector further protects the high-side circuit from an insufficient supply voltage and ensures a sufficient gate-voltage level for the high-side transistor.
Another objective of the present invention is to provide a high-side switch driver with improved noise immunity. This is accomplished by connecting a programmable load in parallel to the input of the comparator. The programmable load provides a variable impedance to prevent noise interference. Furthermore, the two totem-pole buffers produce a differential voltage across the input of the comparator. This differential voltage further strengthens the noise immunity of the high-side circuit, so that it will be suitable for use in high-voltage applications.
To raise the floating voltage and improve the efficiency of the high-side switch driver, the bias voltage charges the bootstrap capacitor when the low-side transistor is turned on. The differential signals also provide additional power via the bridge-rectifier.
The capacitive high-side switch driver according to the present invention overcomes the drawbacks of prior-art high-side switch drivers. In particular, the present invention provides a capacitive high-side switch driver that is suitable for high-voltage and high-speed applications. Moreover, the capacitive high-side switch driver according to the present invention is substantially more efficient and has stronger noise immunity than prior-art switch drivers.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.